1. Field of the Invention
The present invention relates to an image scanning apparatus used for copying machines, facsimiles, and image scanners.
2. Description of the Related Art
In general, image scanning apparatuses used for copying machines, facsimiles, and image scanners include a scanning optical system, a lens unit, and an image sensor. The scanning optical system includes an exposing lamp and a plurality of mirrors. The lens unit includes a lens barrel containing a plurality of lenses. The scanning optical system scans the surface of a document to obtain reflected light. The reflected light is made to form an optical image on the image sensor by the lens unit. The optical image is converted into an electric signal.
Recent improvements in the quality of the scanned image and in the scanning speed necessitate an increase in the amount of light of the exposing lamp and in the driving speed of the image sensor. Due to the heat emitted from the exposing lamp and the image sensor, the temperature inside the image scanning apparatus rises significantly.
The relative position of the document and the lens unit and the relative position of the lens unit and the image sensor are adjusted so as to satisfy a predetermined optical performance when the image scanning apparatus is constructed. However, when the temperature inside the apparatus rises, the relative positions change due to thermal expansion of the lens barrel and a supporting member for supporting the lens unit and the image sensor, and consequently the optical performance deteriorates.
To solve such a problem, Japanese Patent Laid-Open No. 9-049957 discloses an image scanning apparatus in which the focal length of the lens unit at room temperature is longer (or shorter) than that when the temperature is higher than room temperature; and the thermal expansion coefficient of the lens barrel is larger (or smaller) than that of the supporting member for supporting the lens unit and the image sensor.
That is to say, in the above image scanning apparatus, the thermal expansion coefficients of the lens barrel and the supporting member are determined in accordance with the change in the focal length due to the temperature rise. However, if the amount of change in the focal length cannot be compensated by thermal expansion of the members, optical performance deteriorates. Factors such as strength and cost can limit the materials used for the members. In some cases, deterioration in optical performance due to temperature rise cannot be compensated.